The present invention relates to the field of digital memory circuits, and in particular to non-volatile memory circuits suitable for high-density, high-capacity, low cost data storage.
Many consumer devices are now constructed to generate and/or utilize digital data in increasingly large quantities. Portable digital cameras for still and/or moving pictures, for example, generate large amounts of digital data representing images. Each digital image may require up to several megabytes (MB) of data storage, and such storage must be available in the camera. To provide for this type of data storage application, the storage memory should be relatively low in cost for sufficient capacities of around 10 MB to 1 gigabyte (GB). The storage memory should also be low in power consumption (e.g.  less than  less than 1 Watt) and have relatively rugged physical characteristics to cope with the portable battery powered operating environment. For archival storage, data need only be written to the memory once. Preferably the memory should have a short access time (preferably less than one millisecond) and moderate transfer rate (e.g. 20 Mb/s). Preferably, also, the storage memory should be able to be packaged in an industry standard interface module, such as PCMCIA or Compact Flash card.
One form of storage currently used for application in portable devices such as digital cameras is Flash memory. This meets the desired mechanical robustness, power consumption, transfer, and access rate characteristics mentioned above. However, a major disadvantage is that Flash memory remains relatively expensive ($1.50-$2 per MB). Because of the price it is generally unreasonable to use Flash memory storage as an archive device, thus requiring data to be transferred from it to a secondary archival storage. Also, it becomes prohibitively expensive to include a large amount of Flash memory in an inexpensive digital camera or similar digital appliance (MP3 Player, PDA, etc.). This can make certain features unavailable in such applications, for example recording video on an inexpensive digital camera, and can impair the function of other features, for example limiting the number of pictures a digital camera can store or the number of songs which can be stored on an MP3 audio player.
Magnetic xe2x80x9chard discxe2x80x9d storage can also be used for archival storage, even in portable devices. Miniature hard disc drives are available for the PCMCIA type III form factor, offering capacities of up to 1 GB. However, such disc drives are still relatively expensive ($0.5 per MB), at least partially because of the relatively high fixed cost of the disc controller electronics. Miniature hard drives have other disadvantages when compared to Flash memory, such as lower mechanical robustness, higher power consumption (xcx9c2 to 4W), and relatively long access times (xcx9c10 mS). Hard drives also have disadvantages including rotational latency, and xe2x80x98wake-upxe2x80x99 time, which can be one second or more.
Removable optical storage discs can similarly be used, and offer one large advantage compared to hard disc. The removable optical media is very inexpensive, for example of the order of $0.03 per MB for Minidisc media. However in most other respects optical disc storage compares poorly with magnetic hard discs including relatively poor power consumption, mechanical robustness, bulk, and access performance.
Magnetic tape has even lower media cost than removable optical discs, however it shares other disadvantages of rotating disc storage, particularly with respect to physical bulk, and power consumption. In addition, magnetic tape has the disadvantage of serial access. This presents two additional application problems, namely very slow random access performance and restriction to uniform time compression techniques for storing video and the like.
For the specific application of photography, photographic film in the form of silver halide emulsions on plastic webs are a competing form of memory. The drawbacks of conventional film are its need for processing, limited shelf life, and physical bulk. The information storage on photographic film is inherently analogue in nature, and is quite unsuitable for directly interacting with digital processing apparatus and techniques. Furthermore, except in the case of Polaroid(trademark) self developing photographic film, the stored information cannot be accessed immediately.
Embodiments of this invention address the problem of low cost archival storage for digital camera and other portable appliances. The requirements for this type of memory are: an industry standard interface (e.g. PCMCIA or Compact Flash), 2000 G shock tolerance, low power consumption ( less than  less than 1W), short access time ( less than 1 ms), moderate transfer rate (20 Mb/s), and sufficient capacity (10MB-1GB).
In accordance with the principles of the present invention, there is provided a data storage device comprising a cross-point memory array formed on a dielectric substrate material. The cross-point memory array comprises first and second sets of transverse electrodes separated by a storage layer including at least one semiconductor layer. The storage layer forms a non-volatile memory element at each crossing point of electrodes from the first and second sets. Each memory element can be switched between low and high impedance states, representing respective binary data states, by application of a write signal in the form of a predetermined current density through the memory element. Each memory element includes a diode junction formed in the storage layer, at least whilst in the low impedance state.
In one form of the invention the dielectric substrate is a formed from a polymeric material. In another form of the invention, the dielectric substrate is formed from a metal film, such as stainless steel, having a coating of dielectric material thereon.
The dielectric substrate may be formed, for example, from a material selected from: polyimide; polyethersulphone (PES); polyacrylate (PAR); polyetherimide (PEI); polyethylene napthalate (PEN); polyethylene terephthalate (PET); polyester terephthalate; polytetrafluoroethylene (PTFE); polycarbonate; and polyvinyl chloride (PVC);
The storage layer is preferably formed from material capable of processing at temperature lower than the processing temperature of the substrate material. In one form of the invention, the at least one semiconductor layer of the storage layer is formed from an organic semiconductor material. In embodiments of the invention, the organic semiconductor material may be selected from: copper pthalocyanine (CuPc); PTBCI (3,4,9,10-perylenetetracarbonxilic-bis-benzimidazole); PTCDA (3,4,9,10-perylenetetracarboxilic danhydride); BTQBT [(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole)]; TPD (N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)1-1xe2x80x2biphenyl-4,4xe2x80x2-diamine); xcex1-NPD (4,4xe2x80x2-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl); and TPP (5,10,15,20-tetraphenyl-21H,23H-porphine).
In other embodiments of the invention, the at least one semiconductor layer of the storage layer is formed from an amorphous inorganic semiconductor material, such as amorphous silicon or germanium.
The data storage device preferably includes address decoding circuitry coupled to said first and second sets of memory array electrodes, the address decoding circuitry having first and second sets of input lines for addressing the first and second sets of electrodes, respectively. Preferably the first and second sets of input lines are coupled through diode elements to selected ones of the first and second sets of memory array electrodes, respectively.
The data storage device preferably also includes at least one data sense line having diode connections to each of the first and/or second set memory array electrodes.
In a preferred form of the invention the first and second sets of memory array electrodes are formed in separate layers separated by said storage layer, wherein the first and second sets of input lines are formed in the same layers as the second and first sets of electrodes, respectively.
In the preferred data storage device structure the first and second sets of input lines are arranged to cross the first and second sets of memory array electrodes, respectively, separated by said storage layer. The selected diode connections between respective input lines and electrodes are formed by said storage layer at the crossing points thereof.
In a preferred embodiment of the data storage device power supply striping circuitry is provided, coupled to said first and second sets of memory array electrodes. The power supply striping circuitry preferably comprises ends of the electrodes coupled through respective resistive elements and coupled together in groups to selective power supply stripe lines.
A particular preferred embodiment of the present invention comprises a memory module having a plurality of module layers each having a data storage device as recited above.
In one form of the invention the module layers are laminated together to form a block, and electrical contacts are provided on at least one external surface of the block for providing interconnection with the first and second sets of input lines, the at least one sense line and the power supply stripe lines of each module layer. Preferably corresponding input lines from each of the plurality of module layers are connected in parallel to address contacts for parallel addressing of the memory arrays in the memory module.
In accordance with the present invention there is also provided a memory module comprising a plurality of memory circuits formed in a unitary package, each memory circuit comprising a non-volatile cross-point memory array and an address decoding circuit fabricated on a common non-semiconductor substrate.
Each memory circuit is preferably formed from first and second layers of transverse conductor tracks separated by a storage layer including at least one semiconductor material layer. In a preferred embodiment, the memory array of each memory circuit comprises a matrix of memory elements, each memory element being formed by overlying portions of conductor tracks from the first and second layers at a crossing point thereof and a portion of said storage layer therebetween. The memory elements may be switched between low and high impedance states, representing respective binary data states, by application of a write signal in the form of a predetermined current density through the memory element. Each memory element includes a diode junction formed in the storage layer at least whilst in said low impedance state.
According to a preferred circuit structure, the address decoding circuit of each memory circuit comprises selected diode interconnections made through the storage layer between overlying portions of conductor tracks from the first and second layers at crossing points thereof.
A particularly preferred form of memory module is constructed from a plurality of module layers, each module layer having at least one of said plurality of memory circuits. The module layers may be laminated together to form a block, and electrical contacts provided on at least one external surface of the block for providing interconnection with conductor tracks of the memory circuits. In a preferred embodiment, inputs to the address decoding circuits of a plurality of the module layers are coupled together in parallel to the external electrical contacts.
The present invention also provides a digital data memory system comprising: a memory module having a plurality of non-volatile cross-point memory arrays formed in a unitary package; and an interface card adapted for interconnection with a data processing appliance. The interface card includes control circuitry for addressing, writing to and reading from the memory module, and a memory module interface adapted to receive a memory module and interconnect the control circuitry to the memory arrays thereof.
The memory module for use in the digital data memory system can be constructed as recited hereinabove, for example. The memory modules preferably provide permanent data storage, for example in the form of write-once non-volatile memory.